1. Field
The present invention relates to an integrated optical modulator; and, more particularly, to an integrated optical modulator including a carrier wave band modulator using a double sideband modulation method and intermediate frequency band modulator using a single sideband modulation method, which is free from chromatic dispersion.
2. Background
With an explosive increase in demand for wireless communication equipment and information, a new information communication environment is strongly required so as to transmit and receive data of more than hundreds of Mbps. Such broadband transmission necessitates a carrier wave having dozens of GHz. Particular, 60 GHz band, which is readily absorbed by oxygen, is being greatly noticed as a carrier wave in view of frequency recycling.
There are several techniques for transmitting signals to wireless communication equipment. One technique is intermediate frequency (IF) over fiver (hereinafter referred to as “IOF”) or baseband over fiber (hereinafter referred to as “BOF”) which is the way that transmits data to a base station and then modulates a carrier wave with the date signal to transmit a modulated carrier wave in the base station. Another technique is radio frequency over fiber (hereinafter referred to as “ROF”) which is the way that transmits a carrier wave modulated by using a data signal to a base station and then transmits directly radio frequency (RF) regenerated by a high-efficiency photodiode in the base station. Such communication to a base station is carried out by using an optical fiber with broadband and low loss characteristics. Particularly, in a circumstance requiring a large number of base stations due to short-haul, high capacity transmit/receive characteristics of a high frequency, ROF transmission is preferred because in case of the ROF method, designing a base station is relatively simple.
In an ROF transmission system, an optical modulator to convert electric signals into optical signals is essential. To enhance transmission characteristics, various modulating methods and optical modulators corresponding to the modulating methods have been studied and developed.
For example, a first conventional modulation method is to directly vary an RF carrier wave having dozens of GHz into an optical signal using a data signal at an optical modulator. To perform such an optical modulation, a lot of RF signals have to be processed and an optical modulator for a carrier wave band is necessary. However, in case of 60 GHz band, it is difficult to perform high efficiency optical modulation because of high RF loss and further, in case of such modulation, a transmission distance is limited due to chromatic dispersion between double sidebands.
A second conventional modulation method is to use two light waves, wherein a frequency difference between the two light waves is the same as a frequency of carrier wave. In detail, after generation of two light waves so that a frequency difference between the two light waves is the same as a frequency of carrier wave, one light wave is transmitted without modulation, and the other light wave is modulated and transmitted by using a data signal. Then, in a receiving-end, a signal of a carrier wave band is detected by a broadband photodiode and filtered. This method ensures easier modulation compared to the above-mentioned first conventional modulation method because it requires optical modulation for only an intermediate frequency band of several GHz. However, this method has difficulty in continuously generating light waves so that a frequency difference between the light waves is the same as a frequency of carrier wave, and is vulnerable to chromatic dispersion.
A third conventional modulation method is similar to the first conventional modulation method except that the third conventional modulation method can minimize influence of chromatic dispersion by removing one side band of a modulated signal. The third conventional modulation method is known as a single sideband (hereinafter referred to as “SSB”) modulation. The SSB modulation is based on a principle that one side band is offset to be removed through the steps of dividing an RF signal having carrier wave band into two signals, conducting a 90° phase lag for one signal of them, modulating the two signals in a branched optical waveguide part of mach-zéhnder interferometer structure, respectively, and combining the modulated two light waves. However, this method also requires an optical modulator for a carrier wave band and is difficult to apply to an ROF system in spite of its advantages because of complex modulator structure and need of additional RF circuits.
Although various modulation methods and modulators have been studied, the conventional modulation method and modulators could not be easily applied to an ROF system due to imperfect features and complex system constitution. Accordingly, there is need for a new transmission method having simple and improved characteristics and a modulator corresponding to the new transmission method.